This application concerns roll fed thermoforming machines of a well known type which mold products into plastic sheet material fed into the machine from a roll. The plastic sheet is advanced by movement of a pair of drive chains while being supported by a pair of spaced apart parallel rails extending through an oven to heat the plastic sheeting and thence into a forming station where product is molded into the heated sheet by pressure and/or vacuum.
After a cooling interval, the sheet is advanced out of the thermoformer and into a trimmer where the products are cut from the sheet.
Such thermoformers are typically adjustable to be adapted to different sheet widths by moving the rails to the spacing required for a particular sheet width.
A complication is created by the growth in width of the plastic sheeting when heated and also by a tendency of the sheet to sag as the plastic material is softened by the oven heating.
Such sagging can interfere with the molding process so that in the past a portion of the rails were driven apart laterally to eliminate any sag. This motion is resisted by the sturdy rails which must be bent in the region of the oven to produce that lateral movement.
This makes precision fine adjustments difficult due to the resistance to bending of the rails and requires powerful motors to carry out. It can also cause binding in the adjustment mechanism. The long lengths of channel needed for the rails increases the cost of fabricating the machines.
The plastic sheet is engaged by a pair of recirculating chains each supported by one of the rails, which chains have sticker tips which penetrate the plastic sheet on each side to cause the sheet to be moved with the chains as they move along the rails. Both chains have traditionally been driven by a single large electric motor which simultaneously drives a pair of spaced sprockets by means of an indexing system including a common shaft connecting the sprockets.
This insures that each side of the sheet is driven precisely equally to keep the sheet straight with respect to the rails in order to be oriented properly with the molding tooling.
The presence of the cross shaft hinders changing tools and the indexing system itself requires substantial maintenance. A large motor must be used to power driving both chains, and this slows the response time of the drive system.
The drive cross shaft must also be configured to accommodate sheet width adjustments increasing the complexity of the chain drive.
It is an object of the present invention to provide an improved rail system and chain drive to eliminate the difficulties presented by the long length rails in adjusting the rail spacing at points long their length to keep the plastic sheet flat as it is heated.
It is another object of the present invention to simplify the chain drives to eliminate the indexing components and improve its performance.